This invention relates to diamond microelectrodes.
Electrochemistry utilises the relationship between current and voltage measured on immersed electrodes to characterise the solution in which the electrode is immersed. Dependent on the application, one of the current or voltage may be fixed and the other parameter allowed to vary, for example as the solution varies. Alternatively the solution may be essentially fixed, and one of the current or voltage may be swept across a range of values and the response in the other parameter recorded in the form of a time/current plot or voltammogramme.
Electrochemical measurements can be qualitative or comparative, or they may be quantitative. Quantitative measurements generally require that the system is amenable to mathematical modelling, and in both cases it is desirable that the signal to noise in the system is maximised and that as much information as possible is extracted from the system (see Feeney et al, Electroanalysis 200, 12 no. 9). Both of these objectives can best be achieved by using small electrodes, i.e. microelectrodes, such that the configuration approximates to a semi-spherical or three dimensional diffusion model rather than either a linear or two dimensional diffusional model.
The use of such microelectrodes is well known in the art, and became an active field of research in the late 1970's. Subsequent general development of electronics has provided the tools required to utilise such electrodes efficiently. Typical benefits realised include increased temporal resolution, increased current density, decreased sensitivity to solution resistance, and steady state diffusion profiles.
From the point of view of the analysis, the simplest microelectrode is a hemisphere, matching the mathematical geometry of constant concentration surfaces further out into the fluid. However, for a planar disc at large values of Dot/ro2 (where Do is the diffusion coefficient of the species being electrolyzed, t is the time after applying the voltage and ro is the radius of the electrode), a hemispherical diffusion layer can typically be envisioned over the disc and the geometry is still amenable to analysis. For arrays of electrodes in a static system, the behaviour of the system depends on the electrode spacing. For short times, or large electrode spacings, the electrodes behave independently and the total output of an array electrically connected in parallel is the sum of the outputs of the individual electrodes. For long times, or closely spaced electrodes, the individual diffusion profiles overlap and in the extreme the system behaves like a single electrode with a total area of that of the array (sum of individual electrode areas and the intervening insulator). In flowing systems, the characteristic time remains generally short, dependent on the flow rate.
Typically, in order to fabricate such microelectrodes, a conductive electrode material such as a metal is coated with a non-conducting layer which is then perforated with one or more apertures to form the microelectrodes which will come into contact with the solution. More recently, boron doped CVD diamond has become established as an electrode material, and fabrication of microelectrodes onto a boron doped diamond layer has been reported. Typically such electrodes are a few microns in diameter, fabricated by applying a layer of Si3N4 or similar non-conductive material to the surface of the diamond and subsequently etching apertures into it to expose the diamond underneath (e.g. P Rychen et al., Electrochemical Society Proceedings Vol. 2001-23 pp 97-107).